adjust workflow to install mamba-ssm when running tests on torch backend

Author: LarsKue

.github/workflows/tests.yaml

@@ -61,6 +61,13 @@ jobs:
         run: |
           pip install -U torch torchvision torchaudio --index-url https://download.pytorch.org/whl/cpu
 
+      # we currently have extra dependencies for torch tests
+      # TODO: can we install this dynamically? should we add it to the test dependencies?
+      - name: Install Extra Dependencies
+        if: ${{ matrix.backend == 'torch' }}
+        run: |
+          pip install -U mamba-ssm
+
       - name: Show Environment Info
         run: |
           python --version